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Sensors and Actuators A 194 (2013) 106– 111

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

jo u rn al hom epage: www.elsev ier .com/ locate /sna

agnetic actuated pressure relief valve

tefan Gassmann ∗, Lienhard Pagelniversity of Rostock, Faculty of Computer Science and Electrical Engineering, Institute of Electronic Appliances and Circuits, Germany

r t i c l e i n f o

rticle history:eceived 24 May 2012eceived in revised form1 December 2012ccepted 28 December 2012vailable online 30 January 2013

a b s t r a c t

Pressure relief valves are needed in almost any fluidic system. The goal of such a valve is to avoid dan-gerous overpressure inside the channels and cavities. Conventional spring based pressure relief valvesare difficult to integrate in flat micro fluidic systems and small pressure values are difficult to achieve.The here presented solution is a simple flat structure using permanent magnets. The set pressure can beadjusted in a very large range. The suggested solution has faster opening and closing behaviour compared

eywords:ressure relief valveab-on-BoardCBluidics

to spring based valves, is self-aligned and very easy to built. The here presented valve was build in thefluidic printed circuit board technology and works at a set pressure of 300 mBar.

© 2013 Elsevier B.V. All rights reserved.

CBMEMS

. Introduction

The protection against overpressure in the fluidic system is notnly for micro systems a very important task. When an overpres-ure occurs inside the channels sensitive elements can be destroyedr bonds can break and the fluidic system becomes defective. Var-ous reasons for dangerous overpressure are possible, e.g. syringeumps or pumps that try to deliver a constant flow even at highressure; thermal expansion of liquids inside a closed system orrong setup of external pressure sources. Devices that prevent a

ystem from dangerous overpressure should work without the sup-ly of energy. Only if these devices work independently an efficientrotection can be assured. Such elements are pressure relief valvesr safety valves.

Safety valves for protecting MEMS systems are already reported.erdigones et al. [1] describe a safety valve that is made in a PCBEMS process and protects the channel system by blocking the

nlet. An inside generated overpressure cannot be relieved. A siliconased pressure relief valve was fabricated by NanoSpace AB [2]. It

s used as a safety element in a cold gas micro propulsion systemor small satellites. The set pressure is >10 Bar. Passive check valvesith one opening to the environment could also be used as safety

lements. But normally they are designed to work with a minimal

pening pressure that is not desired for safety valves. Spring basedafety valves are widely used in pneumatic macro systems. Thesere the standard implementation of a safety valve. Due to the size

∗ Corresponding author.E-mail address: stefan.gassmann@uni-rostock.de (S. Gassmann).

924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.sna.2012.12.033

and the difficulty to build valves with the set pressure < 1 Bar theyare often not suitable for MEMS devices.

The here presented solution consists of permanent magnets andsimple sealing elements. Using this structure the set pressure canbe defined in a very large range. The valve is easy to assemble; selfaligned and has a better pressure relief characteristic than springbased valves. The reported solution is the first pressure relief valvefor MEMS using permanent magnets, a patent has been granted [3].

In the following the solution is described in detail. Since thestructure is build using a fluidic PCB technology a short introduc-tion is given to this technology. In Section 3 the basics of reliefvalves are described. Section 4 gives an introduction to the mag-net based pressure relief valve, the here presented facts wherealready reported in [4]. In Section 5 the numerical simulation ofthe behaviour and simulations for the setup are depict. The nextsection gives a detailed description about the realization and giveshints for an adaption of the here presented solution to other sys-tems. In Section 7 a detailed description of the tests is given. Thispaper ends with some conclusion remarks.

2. Basics of the fluidic PCB technology

At the University of Rostock a technology to create fluidicsystems based on printed circuit boards was developed. This tech-nology allows hybrid integration of both electronic and fluidic

components on one substrate at low cost. The main advantages forthis technology are: low cost materials; well established and highlyavailable technologies and the seamless compatibility betweenelectronics and fluidics.

S. Gassmann, L. Pagel / Sensors and Actuators A 194 (2013) 106– 111 107

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Fig. 1. Basic principle of a spring based pressure relief valve.

This technology uses printed circuit boards consisting of epoxyaminate (FR4) with copper coating on both sides. These boardsre structured by drilling and milling (for the whole board) and bytching for the copper plating. These boards and the treatment aretandard in the electronic industry and are available at low cost.hannels for High-Flow application can be created by milling ofn inner layer. Small channels for micro fluidics are possible bytructuring the copper lines. A short description of High-Flow andicro fluidic applications is given in [5].

. Basics of relief valves

A pressure relief valve is a device that protects a closed channelystem against overpressure. For this purpose a few technical solu-ions are available. One of them is a pressure relief valve. The mostressure relief valves have an opening from the channel systemo the environment. This opening is closed with a sealing elementhat is pressed to its position by a spring. This spring can be in var-ous forms like flat springs or coil springs. In the most realizationsoil springs are used. If the force generated by the pressure insidehe channel system is higher than the pressure generated fromhe spring, the sealing element is moved and the opening is free.he pressure inside the channel system can be relieved. The forceenerated by the pressure can be easily calculated by the formulap = p · A (with p = pressure inside the system and A = surface of theealing element). The force generated from the spring is also easy toalculate after the formula Fs = D · d (with D = spring constant and

= spring deflection). This realization works fine in a pressure rangever 1 Bar. The systems for smaller pressure have some drawbacksike stick effects and friction of the sealing element [6].

Fig. 1 shows the basic principle of a relief valve. The pressure inside the system generates a force on the sealing element Fp. Ifhis force is smaller than the external force Fext the valve is closed.

hen the pressure generates a force higher than the external forcehe sealing element will be moved and the pressure can be relivedrom the system. This pressure when the valve opens is the setressure. When the pressure inside the system becomes smaller,he external force pushes the sealing element to its place and closeshe opening. In spring based relief valves this closing is slow andas a creep back behaviour [6].

. The magnet based pressure relief valve

.1. Basic principle

The basic principle for the safety valve remains the same: anrifice is covered by a sealing element which is placed to its positiony an external force. In the here presented realization the external

orce will be generated by permanent magnets. This can be twoermanent magnets on both sides of the valve or one permanentagnet and one piece of ferromagnetic material. The principle of

uch a valve is shown in Fig. 2.

ferro magnetic mater ial

Fig. 2. Basic principle of pressure relief valve with permanent magnets.

This leads to a very simple structure. Only two magnets andone sealing element are needed. The advantages of this structureare: simple and flat design, direct pressure relief characteristics,wide range of set pressure feasible, flexible behaviour setup. Theseadvantages are described in the following.

4.2. Design

The here presented design of a pressure relief valve is very sim-ple and flat. Only the magnets and one simple sealing element (likean O-ring) are needed. Other relief valves needs special valve seats,holder and centring parts for the sealing element. This is not neededwhen magnets are used.

For the usage in MEMS technologies a flat structure is advan-tageous. Using coil spring based pressure relief valves long springsare needed. This will lead to an elevated structure. This is disadvan-tageous in MEMS technologies. The here proposed structure usesonly two discs of permanent magnets or one disc of permanentmagnet and one ferromagnetic material and one sealing element.It is flat and can be mounted outside of almost any MEMS system.

The assembly of the pressure relief valve is also very easy. Dueto the attraction force between the magnets both magnets are holdin place. For the assembly the sealing element and the top magnetare joined by glueing, the lower magnet is also put in place on theMEMS system. If the magnets are now close enough together themagnet with the sealing element will move to the right place on theopposite site of the bottom magnet. This is a kind of self-assembly.

4.3. Pressure relief characteristics

The optimal relief valve opens direct at the set pressure. Whenthe pressure in the system is still rising the relief valve opens moreso that the opening gets bigger and the pressure inside the system isnot rising above the set pressure. This behaviour cannot be achievedwith normal springs. The force created by the spring is propor-tional to the spring deflection (see Fig. 3). A wider opened orificecan only be achieved by a higher force, in fact when the valve is onceopened the pressure drops and the generation of a higher force isnot possible only the impulse of the fluid holds the valve open.Using permanent magnets the behaviour of the force between themagnets and the distance is different. It is a highly degressive rela-tion. The force is proportional to 1/d2 (with d = distance betweenthe magnets). That means if the sealing element (with the magnet)is moved, the attraction force will be much smaller. This leads to avery sharp limiting of the pressure inside the system; if the pres-sure relief valve is opened once by a pressure that is equal to the

set pressure, the pressure cannot raise more.

Fig. 3 shows the force generation of a spring and a permanentmagnet in dependence of the distance. This figure illustrates thequalitative difference in force generation. While the spring needs

108 S. Gassmann, L. Pagel / Sensors and Actuators A 194 (2013) 106– 111

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ig. 3. Qualitative comparison of the force generated by a spring and by permanentagnets in dependence of the distance.

ore displacement (d) to generate a higher force. The force gener-ted by the magnet drops with a higher displacement. This leads to

very sharp opening behaviour.The closing behaviour of the magnet equipped valve is faster in

omparison to conventional valves. Spring based relief valves arenown for the creep back behaviour [6]. The full sealing is reachedt a pressure level much lower than the set pressure. Relieve valvesith the magnetic element will seal much faster because when theyove back to the valve seat the force between the magnets will rise

ery fast. So a high sealing force is reached.Another advantage that is also important for the characteristics

s the self-alignment and self-sealing behaviour. Due to the magneteld properties the magnet discs align themselves. For spring basedalves a big effort is needed to hold the sealing element in place.

.4. Range of set pressure

The usage of permanent magnets gives the possibility to create very broad range of set pressures. These include low pressuresnder 100 mBar and also higher pressures up to 10 Bar. The designarameters to change are the magnetization (by choosing differentrades of the magnet), the size of the sealing element and the dis-ance of the magnets. These values are easy to change. To achievehe right values a simulation can be used (see Section 5).

.5. Changing of the behaviour

With the help of the design parameters it is also possible tohange the behaviour of the safety valve. So it is possible to designalves that are not reclosing when the pressure inside the systemas higher than the set pressure. This means when the pressure

s over the set pressure the sealing element will be moved awayrom the opening. Even when the pressure is reduced, the channelystem will stay open. Such behaviour is needed when the systemeeds a service or a check after the accidental high pressure. Whensing a permanent magnet the reset of such a pressure relief valve

s very easy. It is just needed to bring the sealing element with theagnet near enough to the second magnet (or ferromagnetic mate-

ial). The attraction force will align and seal the sealing element bytself.

. Simulation of the force between the magnets

With the help of a numerical simulation the behaviour whenhe valve is opened should be investigated. In a second simulationhe right values for the setup should be found. Both simulations areescribed in this section.

Fig. 4. Meshed model of the two permanent magnets.

As the simulation tool COMSOL 3.4 was used. In a first simula-tion the behaviour when the valve is opened should be investigated.In the moment when the force generated by the pressure inside isbigger than the attraction force between the magnets the uppermagnet begins to float on an air cushion. This kind of movement isunstable and the magnet will move sideward from its position andwill also tilt. The behaviour of the magnet in a sideward movementor when tilted was simulated. For this purpose a model was cre-ated with two magnets with the diameter of 12 mm and the height2.5 mm.

Automatic meshing was used to generate the mesh. Fig. 4 showsthe meshed model. First a simulation was carried out with a verticaldisplacement of 3 mm and a horizontal displacement of 1 mm. InFig. 5(a) the simulation results are displayed. The resulting forcevector points to the centre of the lower magnet. This was theawaited behaviour. The properties of the magnetic field lead to aforce that acts in a manner that the upper magnet will be centred.So the behaviour of the structure is self-centring. In a second simu-lation the case of a tilted magnet should be investigated. When themagnet flows on the air cushion during the open state of the safetyvalve also a tilting can be observed. In Fig. 5(b) the force was simu-lated when the magnet is lifted from the opening and tilted by 10◦.The resulting force vector is now direct in the vertical direction. Butthere is also a torque that will turn the magnet counter-clock-wise.That means the magnet will be tilted even more by the magneticfield. There is no self-levelling behaviour. In the reality the tilt-ing of the upper magnet will be blocked when the sealing elementtouches the surface of the valve seat and the magnet will align onthe surface.

In a second set of simulations the right parameters for theneeded set pressure should be calculated. So the force betweenthe magnets should be calculated. The calculation of the forcebetween permanent magnets is not trivial [7]. The most formu-las use approximations. Tables or thumb rules are available fromthe suppliers of permanent magnets [8,9].

The valve in this application should be designed for a set pres-sure of 300 mBar. The diameter of the sealing element was chosenas 10 mm and for the magnets 12 mm. With the diameter of the O-ring of 10 mm the generated force at the set pressure is about 2.4 N.That means the force generated by the permanent magnets mustbe 2.4 N. If the magnets and the diameter of the sealing element

are chosen, the force can be adjusted by the distance between themagnets. A calculation of the force in dependence of the distanceis needed.

S. Gassmann, L. Pagel / Sensors and Actuators A 194 (2013) 106– 111 109

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O-ring, the more flow can pass through the annular gap between

Fig. 5. Simulation of the

Here the simulation and in parallel the measurement of thettraction force between two magnets are used as methods to findhe right distance between the magnets. The simulation was doneith COMSOL 3.4. A parametrical simulation with the distance d a

hanging variable was carried out. Ten distances between 1 mm and.5 mm where simulated. The measurement was done manuallyith a spring force gauge.

Fig. 6 shows the results of the measurement and the simula-ion of the force between the chosen magnets in dependence of theistance. The influence of the material between the magnets waseglected. To realize a force of around 2.4 N a distance of approx..5 mm is appropriate. Here the value from the simulation wasaken. The distance can be realized by the O-ring cord thicknessnd by the thickness of the MEMS (e.g. fluidic PCB).

. Realization

The solution for the 300 mBar pressure relief valve consists ofwo permanent magnet discs with the diameter of 12 mm and

thickness of 2.5 mm. The material of the disc is a Neodymiumron Boron alloy of the grade N35. The type of the magnets is12x02.5EP-N35, they are available at HKCM Engineering (under

ww.hkcm.de). The lower disc is glued on the channel structureith a double sided PSA (pressure sensitive adhesive). On the other

ide of the magnet an opening exists in the channel. On this openinghe second magnet of the same type is placed. On this magnet an

Fig. 6. Force between two permanent magnets (12 mm diameter, 2

iour of the magnet discs.

O-ring is fixed also by a double sided PSA. The O-ring is in this casethe sealing element. A holder is used to avoid losing the magnet incase of a too high displacement. This holder should be made froma nonmagnetic material. In Fig. 7 an exploded view of the structureis depicted.

Fig. 8 shows a photograph of the pressure relief valve. The sim-plicity of the structure is obvious. For the photograph gold coveredmagnets where used for a better contrast, the parameters of themagnets are the same. As a magnet holder a wired resistor is used.This part can be mounted with a pick and place machine in anautomatic manner.

6.1. Setup procedure

For the usage of the design in another application it is necessaryto find the right design parameters like the magnets and the sealingelement. For finding the appropriate setup for other pressure rangesthe following procedure is suggested.

1. Select the size of the sealing elementAn O-ring that fits to the system should be selected, the bigger the

the O-ring and the valve seat.2. Calculation of the needed force for the desired set pressure

Using the formula Fp = p · A (with p = set pressure and A = surfacearea of the O-ring) the needed force can be calculated.

.5 mm thickness, grade N35) in dependence of the distance.

110 S. Gassmann, L. Pagel / Sensors and Actuators A 194 (2013) 106– 111

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pressure. From around 440 mBar the flow rate rises drastically fromthis point the valve is in a fully opened state. When the Lorch valveis closed at decreasing pressure a large hysteresis can be seen. At

Fig. 7. Principle arran

. Selection of the magnetsUsing the formulas given by the magnet distributors or witha numerical simulation like shown in Section 5 select a mag-net that generates the needed force at the distance given bythe MEMS system thickness + O-ring cord size. Since the mag-nets lose their magnetic properties at high temperature selecta magnet with a working temperature higher than the highesttemperature of the MEMS.

. Fine tuneBy changing the cord size of the O-ring or by adding spacersunder the bottom magnet adjust the set pressure to the desiredvalue.

The setup procedure for different set pressures is an optimi-ing problem with several degrees of freedom. So the size of the-ring, the magnet and the distance can be varied for achieving

he right solution. With the hints given here one should come to working solution. A more sophisticated procedure can be devel-ped only when some design parameters are fix e.g. limited numberf magnets.

. Experiments

With the prototype some test measurements where performed.he classification of safety vales is normally done with a diagramift of the sealing element vs. pressure both values are normalized

6]. Even in big safety valves these curves are difficult to measure.nstead the diagram flow rate vs. pressure is used here which is alsoppropriate due to the proportional behaviour between flow ratehrough the safety valve and the lift of the sealing element.

Fig. 8. Photo of a prototype.

t of the relieve valve.

Fig. 9 shows the setup of the test. A manual pressure regulatoris used to change the input pressure. The input of the regulator wasconnected to a compressor with a pressure of 6 Bar. The outputof the pressure regulator goes to the flow sensor AWM5104 fromHoneywell. As a display for this sensor a voltmeter was used. Fromthe flow sensor the tubing goes to the unit under test (UUT) and asclose as possible to the inlet of the UUT the pressure sensor of typeGreisinger GDH 14 AN was connected. The short connection fromthe pressure sensor to the UUT is needed to avoid measurementerrors due to the pressure drop over the tube at higher flow rates.

The test measurements where performed in the following man-ner: first the pressure is raised slowly up to the point where a flowwas measured. This is the set point of the valve. Then the pressurewas raised faster and the flow rate was monitored to record the flowrate in dependence of the pressure, after the pressure was loweredagain to identify the closing behaviour of the valve and to see a pos-sible hysteresis. Two different valves where tested: (1) Lorch safetyvalve type 2122 and (2) permanent magnet based safety valve.

In Fig. 10 the characteristics of the two tested valves are shown.The Lorch type 2122 is a commercially available safety valve fora pressure under 1 Bar. The Lorch valve shows the first leakage ataround 310 mBar. The flow rate rises only slow with the rising of the

Fig. 9. Test setup.

S. Gassmann, L. Pagel / Sensors and Ac

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Lienhard Pagel has been professor in Microsystems and director of the “Institute of

Fig. 10. Flow over pressure curve of different safety valves.

00 mBar (far under the set pressure) the valve is fully closed (thelow closing is referred as the creep back behaviour). The magnetafety valve opens at 300 mBar the fully opened state is reached at40 mBar. From 340 mBar on the flow rate rises with a high slopeith rising pressure. According to the theory described in Section

.3 this slope must be almost vertical. The slope shown here corre-ponds to the pressure drop due to tubing and fluidic channel fromhe pressure gauge to the valve. The hysteresis of the magnet basedalve can be neglected at 290 mbar a fully sealing can be observed.t can be seen that the magnet based safety valve has a faster open-ng and a faster closing compared to the commercially safety valve.he hysteresis is much smaller.

During the tests it could be seen that the upper magnet wasoating on an air cushion as expected. Misalignment that wasdded by hand was corrected by the magnetic force when the valveas opened by an over pressure and the magnet floats back to its

riginal position. Tilting of the magnet as investigated in Section could also be observed. Since the movement on the air cushionuring the pressure relief is very unstable the magnet tilted ran-omly. But when it moves back to the valve seat the O-ring blockshe tilting movement.

With these experiments it could be seen that the magnet basedafety valve has a better characteristic compared to the com-ercially safety valve. The expected and investigated effects of

elf-alignment and tilting were observed and do not prevent theafe operation of the valve.

. Conclusion

The here presented pressure relief valve is simple in design, easyo build and has outstanding properties. For the creation of the here

resented valve only two permanent magnets and a sealing ele-ent are needed. Due to the attraction force between the magnets

he mounting is easy and the structure is in mounting and in theperating mode self-aligned.

tuators A 194 (2013) 106– 111 111

Numerical simulations with COMSOL were carried out to proofthe behaviour of the relief valve. A wide range of set pressure canbe achieved by the variation of the diameter of the sealing ele-ment, by the selection of the magnetic properties of the magnetsand by the distance between the magnets. The right parameterscan be found with the help of a numerical simulation. The proce-dure to adapt a safety valve to a given set pressure is given in thispaper.

The biggest advantage over other solutions is the openingbehaviour. Due to the characteristics of the force between two per-manent magnets the pressure relief characteristic, the opening andclosing behaviour outperform commercial safety valves in the setpressure range < 1 Bar.

It is also possible to realize non-reclosing pressure relief valves.This behaviour is needed when a service should take place after theaccidental over pressure.

The proposed structure is especially adapted for the use in thefluidic printed circuit board technology, where PCBs are used tocreate the channels. But this structure is also usable in any fluidicMEMS technology where it is possible to realize openings on oneside of the system and where non-magnetic materials are used.

References

1] F. Perdigones, J.M. Moreno, A. Luque, C. Aracil, J.M. Quero, Safety valve in PCB-MEMS technology for limiting pressure in microfluidic applications, in: 2010IEEE International Conference on Industrial Technology (ICIT), 14–17 March,2010, pp. 1558–1561.

2] NanoSpace product information, NanoSpace AB, Uppsala, Sweden, availablefrom: http://www.sscspace.com/components (accessed 05.09.11).

3] S. Gassmann, L. Pagel “Circuit Board having a Pressure-Relief Valve, Insufflator”,2012, Worldwide Patent Number: WO002012098257A2.

4] S. Gassmann, L. Pagel, Pressure relief valve with permanent magnets, in:IECON 2011, 37th Annual Conference of the IEEE Industrial Electronics Soci-ety, Melbourne, Australia, 7–10 November, IEEE, 2011, ISBN 9781612849713,pp. 4074–4077.

5] S. Gassmann, L. Pagel, Fluidic systems in printed circuit boards, in: IEEE Interna-tional Symposium on Industrial Electronics 2009, Seoul, Korea, 5–8 July, 2009.

6] B. Nesbitt, Handbook of Valves and Actuators: Valves Manual International, Else-vier, Oxford, 2007, ISBN 978-1-85617-494-7.

7] P.F.W. Preece, The force of interaction between permanent magnets, PhysicsEducation 5 (1970) 275.

8] MS-Schramberg GmbH & Co KG, web page – magnetic field calcu-lation, available from: http://www.magnete.de/en/products/magnetic-field-calculation/holding-force.html (accessed 02.09.10).

9] HKCM Engineering, HKCM product catalog, available from: www.hkcm.de(accessed 05.09.11).

Biographies

Stefan Gassmann is a research associate at the University of Rostock since 2003. Hismain research topics are the non-electric functionalities of printed circuit boards,especially the fluidic ones. In 2009 he received his Ph.D. for his work on a micro flowinjection analysis build in PCBs.

Electronic Appliances and Circuits” at the University of Rostock since 1994. Prior tothis he worked in research and development in the semiconductor industry for 10years. Since 1994 his main topic has been the realization of microfluidics systemsin PCB technology.